Explanation:
The remnants of
nuclear reactors nearly two billion years old were
found in the 1970s in Africa.
These reactors are thought to have occurred naturally.
No natural reactors exist today, as the relative density of
fissileuranium has now decayed below that needed for a
sustainable reaction.
Pictured above is Fossil Reactor 15, located in Oklo,
Gabon.
Uranium oxide remains are visible as the yellowish rock.
Oklo by-products are being used today to
probe the stability of the
fundamental constants over cosmological
time-scales and to develop more effective means
for disposing of human-manufactured
nuclear waste.